The present invention relates generally to electronic hearing aid control circuits and more particularly to a power level control circuit for a hearing aid amplifier.
Generally, hearing aids incorporate a microphone as an input transducer to receive an acoustic input (sound) and create an electric signal in response. (Alternatively, they may also use an induction coil or a direct electrical connection to produce an input signal.) The electric signal is amplified and transmitted to a receiver that responsively creates an acoustic output (sound) to be heard by the wearer of the hearing aid. For normal sounds, the acoustic output should be a clear but amplified copy of the input signal.
Hearing aids are normally equipped with a gain, or "volume," control. Such a gain control allows the wearer to adjust the amplification of the hearing aid in order to accommodate his needs.
In addition, it is desirable for the hearing aid to limit its output signal for loud input sounds so that the resultant output sound can never become so loud as to be uncomfortable to the wearer. Individuals differ greatly, however, in the sound level that they can comfortably tolerate. Therefore, the level at which the hearing aid limits its output signal should be adjustable for each individual wearer.
A control used to make such an adjustment of maximum output level is often referred to as a power level control. This control is normally adjusted by the person fitting the hearing aid and is not meant to be adjusted by the wearer during use.
Such adjustment typically involves noting the sound level at which the wearer experiences discomfort. This is often referred to as the wearer's loudness discomfort level or tolerance level. The hearing aid power level control is then adjusted so that the hearing aid will not cause a discomforting acoustic output for the wearer, regardless of how large the input signal is.
It has been found that persons with lower loudness discomfort levels normally also require less amplification. Accordingly, it is desirable that adjustment of the power level control will also cause the hearing aid gain to be adjusted. Thus, it is desirable for one adjustment of the pwer level to necessarily cause the gain to be changed to a level that is approximately correct for a particular user. Consequently, the user-operated gain control will be in the appropriate range and require only limited adjustments by the user.
Hearing aids must, of course, provide the wearer with accurate sound reproduction. Thus, it is desirable that the power level control adjust the operating conditions in the hearing amplifier so that, for any setting of the power level control, the hearing aid produces a clear (or "nondistorted") acoustic output signal, as long as the output signal is below the level that will cause discomfort for the wearer.
In addition, it is desirable that a hearing aid require a low level of electric power in operation. Otherwise, a battery that powers the hearing aid will become depleted quickly, requiring frequent replacement. Therefore, it is desirable that the power level control adjust the operating conditions of the hearing aid so that, for any setting of the power level control, the electric power drawn from the battery is low. At the same time, however, the control must maintain the hearing aid in a condition such that it will produce a nondistorted acoustic output signal for normal acoustic input levels.
For ease of use by the person fitting the hearing aid, it is desirable that the hearing aid allow the power level, gain, and operating conditions to be adjusted simultaneously by the operation of a single manual control. Moreover, it is important that the cost of manufacture, and therefore the price to consumers, be kept low. Thus, the power control circuit should preferably require only a small number of relatively simple components.